On the next page look for the box labelled Request API signature. Make sure the radio button in there is checked, and then click Agree and Submit.

On the next page you'll have a list of the

API Username

API Password

Signature

each with a Show link next to it. Clicking Show will will reveal the proper string for each item.

Entering Keys in EDD Settings

Once you have those three items, in the WordPress admin go to Downloads → Settings → Payment Gateways → PayPal Standard and you'll find fields to enter them in.

Getting Test API Keys

Getting Test API keys is documented in our PayPal Sandbox document. Take the Seller keys created there and enter them into the Test API fields you find under the Live API fields on the settings page mentioned above.